The present invention relates to a process and a system for measuring the course of a signal at a point of measurement on a sample with the aid of a corpuscular probe.
Usually checking the functioning of high integrated circuits occurs in computer-controlled semi-conductor test systems, in so-called function testers, in which errors in the integrated circuit are recognized by means of an analysis of the voltage level measured at the outputs of the examined circuit in dependence on the input bit patterns, but can be localized only with great difficulty. For this reason, additional checks and measurements have to be conducted inside the highly integrated circuit, especially during the development phase.
Corpuscular beam measuring processes, employed in all fields of development and fabrication of micro-electronic components, have proven to be particularly suitable for this purpose, especially the electron beam testing technique. In these measuring techniques, a finely focussed corpuscular beam is aimed at the point of measurement and the secondary corpuscles, forming a derived secondary signal, and which are influenced by the course of the signal to be measured at the site of measurement, are registered. A survey of the presently utilized testing procedures ln electron beam testing is found in the publication "Electron Beam Testing" by E. Wolfgang (in the periodical "Microelectronic Engineering", Vol. 4, 1986, pp. 77-106). One of the most important measuring procedures in electronic beam testing is so-called waveform measurement, which is described in detail in the publication "Electron Beam Testing: Methods and Applications" by H.P. Feuerbaum (in the periodical "Scanning", Vol. 5, pp. 14-24), especially on pages 12 to 14, and with the aid of which the voltage waveform at the point of measurement can be measured.
In waveform measurement, a finely focused electronic beam is aimed at the site to be examined in the integrated circuit. The primary electrons impinging there release secondary electrons from the surface of the sample, which are in turn influenced by the electrical potential on the surface of the sample. This influence manifests itself in a secondary electron current, which depends on the potential at the site of measurement, i.e., on an energy shift of the secondary electrons, which also is determined by the electrical potential at the site of measurement and which can be measured with the aid of an energy spectrometer. This effect is referred to as voltage contrast. As the detectors required for registering the secondary electrons usually only have a relatively small band width of a few MHz, a sampling process, in which the temporal course of the signal is sampled for a triggering event at the site of measurement like with a sampling oscilloscope with short electron pulses, must be employed in order to obtain high time resolution. As each primary electron pulse can only contain very few electrons, the sampled values of very many measuring cycles must be averaged in order to obtain a sufficient signal-to-noise ratio, which can result in very long measuring cycles. Therefore, this process is only suited for examining periodically recurring signals.
As in waveform measurement, an electron pulse is generated by the triggering event, the frequency of the triggering determining the required measuring time. For this reason, waveform measurement only yields relatively inexact results at low triggering frequencies, if overly long measuring times are not tolerated. This is extremely disadvantageous of the integrated circuit is to be operated in the same test cycle as in the function tester in order to reproduce the error, for the test cycles of the function tester are usually relatively long and, therefore, only recur with very low frequency. Moreover, only processes that occur after the triggering event can be examined. On the other hand, very often, however, it is the processes occurring prior to the triggering event that are of particular interest. This is the case, by way of illustration, if the integrated circuit is to be controlled by a semi-conductor tester and the output signal of the function tester, which indicates whether or not the component is operating properly, is to be employed as the trigger signal. The triggering event in this example would be the occurrence of an error in the operation of the integrated circuit, by way of illustration the erroneous readout of a memory cell. It is readily understandable that in this case the course of the signal at the respective measuring site prior to the triggering event, which permits drawing conclusions about the source of the error, is of particular interest. Although, in principle, the electron pulse can be generated with the trigger signal from one cycle for the next cycle and, therefore, apparently measurements can also be conducted prior to the triggering event, this, however, quite often leads to jitter problems, especially in long test cycles. Moreover, only periodically recurring processes can be examined with this procedure. On the other hand, this procedure is not successful with, often recurring processes occurring only statistically distributed. Such only statistically occurring processes can be caused in integrated slits, by way of illustration, by the impingement of particles. Furthermore, such processes often occur if the integrated circuit is driven to the limits of its capacity (so-called "marginal test") and very many factors that are very difficult to influence effect the function of the component. In all the above-described cases, conventional waveform measurement is not successful.
An object of the present invention is to provide a process and a system for measuring the course of a signal at a point of measurement, with the aid of which courses of signals only occurring with very low frequency of recurrence and, moreover, under circumstances, occurring only statistically distributed, can also be measured. Another object of the invention is to provide a system which can examine processes occurring prior to the triggering event.